1. Field of the Invention
This invention pertains to inductive plethysmographic transducers of the type used in apparatus for monitoring body functions, such as respiration and heart volumes. More particularly, the present invention pertains to improvements in the design of such transducers and the electronic circuitry associated therewith.
2. State of the Prior Art
Commonly assigned U.S. Pat. No. 4,308,872 entitled METHOD AND APPARATUS FOR MONITORING RESPIRATION, the contents of which are incorporated herein by reference, discloses a particular respiration monitoring apparatus. The apparatus disclosed in the patent employs two electrical conductors, e.g., wires, one disposed in encircling relation about the chest and the other disposed in encircling relation about the abdomen. Each conductor comprises the inductance element of an LC oscillator circuit of fixed capacitance. Consequently, the frequency of each oscillator varies solely in response to changes in the inductance of its respective conductor which, in turn, varies in response to changes in the cross-sectional area encircled by the conductor. Therefore, in the case of respiratory applications, as the subject breathes the frequency changes at the outputs of the oscillators continuously indicate the extent of expansion and contraction of the subject's chest and abdomen. As more fully explained in said U.S. Pat. No. 4,308,872, by appropriate processing and calibration of these signals, significant respiration data may be obtained. In the art, each of the conductors and their supporting structure is referred to as an inductive plethysmograph. The assignee hereof markets such inductive plethysmographs under the trademark Respiband.TM..
As noted, each inductive plethysmograph requires an LC oscillator and associated circuitry. As presently marketed by the assignee hereof, the LC oscillator and its associated circuitry are housed in a plastic module removably secured to the inductive plethysmograph. Wires extending from the module connect the oscillator circuitry with a remote housing containing circuitry for processing, calibrating and displaying, the frequency changes detected by the oscillator and for displaying the results. Typically, the length of the cable connecting the oscillator module on an inductive plethysmograph with the remote housing is about ten feet. The oscillator module as well as an improved inductive plethysmograph are disclosed in commonly assigned U.S. Pat. No. 4,807,640 entitled STRETCHABLE BAND-TYPE TRANSDUCER PARTICULARLY SUITED FOR RESPIRATION MONITORING APPARATUS, the contents of which are incorporated herein by reference.
Although the arrangement of U.S. Pat. No. 4,807,640 represents an improvement over the prior art, particularly insofar as the oscillator circuitry is simplified and the system is rendered more convenient to use, it too is not without drawbacks. For example, it has been observed that untrained personnel often damage the oscillator modules by pulling vigorously on their wire leads. Moreover, when applied to infants and newborns, even the smallest oscillator modules are inordinately bulky and potentially uncomfortable, as when the infant or newborn is lying thereon.
The ideal solution would be simply to move the oscillator circuity to the remote housing, with the inductive plethysmograph connected to the oscillator circuitry by a wire cable. Unfortunately, this results in a substantial loss of signal, typically about 90%. In this regard, it will be recalled that the electronic parameter which forms the basis for all measurements is the change in the inductance of the inductive plethysmograph. A typical inductive plethysmograph has an inductance of only one to two microhenries, and obviously the changes being detected are substantially less than that. However, a ten foot length of standard coaxial cable, which is approximately the length of cable that would be required to connect the inductive plethysmograph to the remotely housed oscillator circuit, typically has an inductance of several times the inductance of the inductive plethysmograph. As the inductance of the wire cable is in series with the inductance of the inductive plethysmograph, it will be apparent that with a remotely housed oscillator circuit, changes in the inductance of the inductive plethysmograph will represent a relatively small percentage change in the combined inductances of the inductive plethysmograph and wire cable. This is to be contrasted with the situation where such inductive changes are measured relative to the inductance of the inductive plethysmograph alone, as is the case in the arrangement disclosed in U.S. Pat. No. 4,807,640, wherein the oscillator circuit is secured directly to the inductive plethysmograph.
It is accordingly an object of the present invention to provide an improved system comprising an inductive plethysmograph and oscillator circuit therefor wherein the oscillator circuit may be housed remotely from the inductive plethysmograph without a substantial signal loss.
It is a further object of the invention to provide a system wherein the oscillator circuitry may be housed remotely from the inductive plethysmograph and signal loss is reduced by improving the circuit design.
It is yet a further object of the invention to provide a system wherein the oscillator circuit may be housed remotely from the inductive plethysmograph and signal loss is reduced by improving the design of the inductive plethysmograph itself.